In Situ Mechanical Testing of Biological and Inorganic Materials at the Micro- and Nanoscales
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Testing of Biological and Inorganic Materials at the Micro- and Nanoscales C. Eberl and T. Saif, Guest Editors
Abstract A central goal of materials science is to reveal how a material deforms under mechanical stress and how the deformation is related to its microstructure. This goal is best achieved by “seeing” the evolving microstructure when the property is measured quantitatively. Mechanical testing methods have thus evolved over time to test materials at the micro- and nanoscale while observing the changes in the specimen. Recent advances in microtechnology offer a new generation of microscale sensors and actuators that allow in situ studies of both living and nonliving materials in analytical instruments. Such experiments are providing new and fundamental insights on the structure-property relations in materials and revealing remarkable links between the mechanical properties of living cells and their functions. This issue presents five articles that discuss state-of-the-art methodologies for in situ mechanical tests and highlights new findings through the use of these techniques.
Introduction In situ is the Latin expression for “on the spot” or “at the site” and is used in science and engineering in various contexts. In materials science, however, it often means testing a material while “seeing” its microstructure. Early in situ experiments in scanning and transmission electron microscopy (TEM) in the late 1960s and early 1970s showed that it is possible to directly observe deformation behavior under mechanical stress.1,2 Nowadays, it is possible to handle material samples and observe their inner structure over size scales ranging from the micro- to nanometer regimes and also to observe any changes in this structure while conducting load and displacement measurements on the material. This allows, for example, quantitative com-
pression and tensile tests to be performed on biological materials3 with diameters of microns or on metal wires4 with diameters of down to 40 nm while observing the evolving defect structure. The introduction of new experimental techniques is paralleled by continually increasing computing power. This enables novel data analysis techniques that are needed, for example, to calculate 3D reconstructions from TEM5 or x-ray data6 in a short time. Furthermore, this renders post-processing unnecessary, allowing real-time, full-field digital image correlation techniques to analyze complex strain fields while conducting an in situ experiment.7 Material properties such as strength depend on intrinsic8 and extrinsic9–11 size
MRS BULLETIN • VOLUME 35 • MAY 2010 • www.mrs.org/bulletin
scales. The need to explore the size effect on materials behavior has been prompted by dramatic miniaturization in electronics and the need to understand and predict failure due to fatigue12 and thermomechanical loading. Novel synthesis and discovery of nanostructures such as nanotubes and nanowires has further prompted this need. The nanostructures show ultrahigh strengths,13 which draw attention to the question o
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